1. Field of the Invention
The present invention relates to an optical pickup including an optical element and more particularly to an optical pickup capable of obtaining suitable track error signals during reading and/or writing for multiple optical recording media having different track pitches, and an optical data processing device using the optical pickup.
2. Discussion of the Background
An optical pickup typically has a structure which detects focus error signals and track error signals and controls the position of an objective lens by using these error signals to correctly irradiate a predetermined recording track in an optical recording medium. With regard to the detection of track error signals, a 3-spot system, a push-pull system, and a differential push-pull system (hereinafter referred to as the DPP system or simply DPP) are typical well known examples.
In particular, the DPP system uses a relatively simple optical system with highly sensitive track error signal detection. In addition, the DPP system has an advantage in that it can detect relatively reliable track error signals in which any offset of the track error signals ascribable to displacement of the objective lens or tilt of the optical recording medium is suitably removed.
Track error signal detection using the DPP system is briefly described. An optical pickup which employs the DPP system includes, for example, a diffraction element 23 arranged between a semiconductor laser 1 as the light source and a half mirror 26 as illustrated in FIG. 24. The diffraction element 23 includes, for example, straight grooves engraved in the surface thereof at a constant pitch, that is, with a regular, uniform gap between the grooves as illustrated in FIG. 25, and has a function of splitting a light beam emitted from the semiconductor laser 1 by diffraction into at least three light beams, i.e., + or − one dimensional light beam, and zero dimensional light beam.
These three light beams are independently focused by way of the half mirror 26, a collimate lens 5, and an objective lens 29 to form three focus spots 100, 101, and 102 on the signal recording face of an optical recording medium 30 as illustrated in (a) of FIG. 26A. The irradiation positions of these three spots 100, 101, and 102 are adjusted by, for example, controlling the rotation of the diffraction element 23 around the optical axis such that an irradiation position interval δ in the radial direction of the optical recording medium 30, i.e., the direction perpendicular to a guiding groove 31 provided in a cyclical manner on the recording surface of the optical recording medium 30, is substantially equal to ½ of the pitch TP of the guide groove 31 (hereinafter, this guide groove pitch TP is referred to as track pitch). The reflected light beams from the focus spots 100, 101, and 102 on the optical recording medium 30 reach the objective lens 29, the collimate lens 5 and the half mirror 26 again. A portion of the reflected light beams transits the half mirror 26 and enters a light reception element 12 via a detection lens 11.
The light reception element 12 has reception portions 20a, 20b and 20c, which are three half- or quarter-reception portions. The reflected light beams of the optical recording medium 30 independently strike into the predetermined reception surfaces of the reception portions 20a, 20b, and 20c to form detection light spots 200, 201 and 202. The photoelectric conversion signals from these reception surfaces are subjected to subtraction treatment by subtractors 50a, 50b, and 50c to detect the track error signals (hereinafter referred to as push-pull signals) by the push-pull signal system.
The detected light spots corresponding to the main focus spot 100 and the sub focus spots 101 and 102 focused on the recording medium 30 are represented by the detected light spots 200, 201 and 202, respectively. The push-pull signals obtained from the detection spots 200, 201 and 202 are represented by Sa, Sb and Sc. From the relative positions of the focus spots 100, 101, and 102 on the optical recording medium 30, it is apparent that the push-pull signals Sa, on the one hand, and Sb and Sc on the other are about 180° out of from each other. With regard to the push-pull signals, Sa and Sb, and Sa and Sc, are output with the signal waveforms reversed (Sb and Sc are the same phase). Therefore, when the addition signal of the signals Sb and Sc is subtracted from the signal Sa, the signal component is not negated but on the contrary is amplified.
On the other hand, displacement of the objective lens 29 or tilt of the optical recording medium 30 causes a predetermined off set component in each push-pull signal. This offset component is obviously independent of the exact focus spot positions on the optical recording medium 30 and occurs to Sa, Sb, and Sc with the same polarity. Therefore, the offset components contained in each push-pull signal selectively cancel each other out in the subtraction treatment described above. As a result, only the offset component is completely removed or significantly reduced so that a good track error signal can be detected.
That is, for example, the push-pull signals Sb and Sc of (b) of FIG. 26 are added by an adder 51 and the signals thereafter are suitably amplified by an amplifier 52 followed by subtraction treatment from the push-pull signal Sa of the main optical spot 100 by a subtractor 53. Therefore, the offset component contained in the push-pull signal Sa is completely removed or significantly reduced, which leads to output of a suitably amplified track error signal.
As briefly described above, the DDP system can detect track error signals with a high degree of sensitivity by using a relatively simple detection optical system to completely remove or significantly reduce the offset component ascribable to the displacement of an objective lens or the tilt of an optical recording medium.
However, the DPP system involves the following practical disadvantages. As described above, the redial-direction irradiation position interval δ for the three focus light spots is required to be adjusted to ½ of the track pitch TP of the optical recording medium 30. Therefore, suitable track signal error signals are not detectable for the optical recording medium 30 having a track pitch Tp widely outside the range of double of the irradiation position interval δ of the focus light spots.
As increasingly popular optical recording media using a blue ray wavelength (λ=405 nm), for example, there are Blu-ray (hereinafter referred to as BD) optical recording media and HD-DVD (hereinafter referred to as HD) optical recording media; there is also a technology involving an optical pickup which is capable of playing both kinds of optical recording media. However, the track pitch of the BD optical recording media is about 0.32 μm while that of the HD optical recording medium is 0.40 μm. FIG. 27 is a schematic diagram illustrating the optical system of the technology to deal with these two different kinds of optical recording media. As shown in FIG. 27, the optical system has a structure in which one objective lens 9 for HD media focuses the light beam emitted from the blue color light source (semiconductor laser 1b) on an HD optical recording medium 10 and, another objective lens 14 for BD media, on an BD optical recording medium 15. A diffraction element 15 for DPP is located on the light path between the light source 1b and both objective lenses 9 and 14.
In this structure, the positional intervals between the three focus light spots which are adjusted to detect the optimal track error signals for an HD optical recording medium are not suitable for a BD optical recording medium, meaning that the track error signals are difficult to detect in the case of the BD optical recording medium by the DPP system. That is, suitably detecting the track error signals of optical recording media having different track pitches with one optical pickup using the DPP system is difficult, depending on the combination of optical recording media.
One plausible approach to dealing with this issue would be to provide another diffraction element for BD optical recording media on the upstream side of the diffraction element 24 while the diffraction element 24 is used for HD optical recording media. However, in this method, an unused sub-spot is formed on the optical recording medium so that the track error signals are not suitably detected.
As another method, a grating can be provided on the downstream side of a prism 25. However, arranging such a grating is not easy due to layout limitations.